Perhaps there is already a topic on this, but I was curious if there is any advantage or benefit for raising a pet from lvl 1 to max level or do you get the same stats on the pet if you tame a lvl 23 pet and raise it to lvl 25. I have quite a few pets from before MOP from collecting and just started on the battle pets. My darkmoon tonk started a lvl 1 and seems a bit more beefy than the other lv1 21 pets that I have.

No, it's been checked out fairly thoroughly. There's a multiplier for the quality of the pet (preset with pets like the tonk that only come in one quality), sometimes a little leeway in specs (look at the Snow Cub, for example; three spec "settings"), but it doesn't matter if you get a level-1 skunk in Elwynn Forest, a level-11 skunk in the Hinterlands, a level-22 skunk in Twilight Highlands, etc., it'll have the same specs at 25 (assuming they're all the same "base" spec set).